Dehydrocyclization of hydrocarbons



United States PatentO 2,901,518 I DEHYDROCYCLIZATION F HYDROCARBONS John H. Raley, Walnut Creek, Calif assignor to Shell Development Company, New York, N.Y., a corporation of Delaware No Drawing. Application February 18, 1955 Serial No. 489,302

16 Claims. (Cl. 260-'673.5)

This invention relates to an improved process of ae hydrocyclization of hydrocarbons. More particularly, it relates to an improved process for the production of aromatic hydrocarbons for aliphatic hydrocarbons.

The art is replete with various proposals and com' mercial methods for the dehydrogenation of organic compounds, particularly of hydrocarbons of various types, as well as other classes of organic compounds. It is already known that the pyrolysis at high temperatures of various aliphatic hydrocarbons many times leads to the posed and some are utilized commercially forthe pro-j duction of aromatics from aliphatic hydrocarbons. These' catalysts are often provided with acidic characteristics whereby they are also eifect isomerization, such as the isomerization of alkylcyclopentanes to c'yclohexanes,

which are then converted to aromatics.- However, even f these processes which are utilized on'a commercial scale are subject to the usual disadvantages ofheterogeneous phase catalyses, such as decrease in activity of catalyst,

which necessitates, eventually, replacement 'or regeneration, and difiiculties of providing uniform contact between the various portions of the heterogeneous catalystand the hydrocarbon.

It is an object of this invention to provide an improved process for dehydrocyclization of hydrocarbons, and particularly for the production of aromatic compounds. A

further object is to provide an improved process for the production of aromatic hydrocarbons from aliphatic hy-- drocarbons. A more specific object is the production of aromatic hydrocarbons from aliphatic hydrocarbons which contain a chain of at least six carbon atoms and especially from saturated aliphatic hydrocarbons. Some specific objects of the invention are to produce benzene from normal hexane, toluene from normal heptane, toluene from 2- or B-methylhexane, and xylenes from octanes which contain a chain of at least six non-quaternary carbon atoms. A further specific object is to provide an improved process for the production of para-xylene from 2,5-dimethylhexane. It is also an object of the invention to provide an improved process for the production of fused-ring polycyclic aromatic hydrocarbons from aliphatic hydrocarbons, such as the production of naphthalene from n-decane, and from arylalkanes containing six contiguous carbon atoms, no more, than two of which are in a single ring, such as the production of naphthalene from n-bntylbenzene, from o-diethylbenzene, and the like.

It has now been found that cyclization of hydrocarbons containing at least six contiguous carbon atoms, no

more than two of which are already in a single ring, can be effected in an efficient and improved manner by subjecting a mixture of such a hydrocarbon and a substantial proportion of free iodine to an elevated temperature suflicient to effect a C-to-H bond cleavage in the molecule in the presence of free iodine. The iodine can be supplied as iree iodine or as an iodine compound which yields free iodine, either molecular or atomic, under the reaction conditions. References hereinafter to iodine are meant to include equivalent amounts of such compounds, such as alkyl iodides, e.g. ethyl iodide, propyl iodide, hexyl iodide, and the like.

Although molecular iodine generally has been regarded as unreactive' toward CH linkages, particularly in bydrocarbons and especially in the case of saturated aliphatic hydrocarbons, it has now been found that, .at elevated temperatures which are ineffective alone to efiect any appreciable C-to-H or C-to-C bond cleavage in the particular hydrocarbon, heating of a substance containing at least six-contiguous carbon atoms not in a single ring, for a veryshort time, in the presence of a substantial proportion of free iodine, elfects bond cleavages and the formation of new C-to-C bonds as are'necessary to provide'the hydrogen deficiency for'the ring formation.

For instance, when a vaporous feed mixture of n-hexane and about l; 9-rnol proportions of free iodine is passed through areaction zone at about 497 C., at atmospheric pressure and'for a nominal residence time of about 37 seconds,"a conversion 'of about 58% of the hexane results,'-- with a yield of about mol percent benzene, basedon the molsof hexane converted. The hydrogen removed from the hexane is accounted for in the reaction product'essentially ashydrogen iodide. In like manner, n-heptane has been converted to toluene in good conversion andyield. The other heptanes, which contain a 6-C=chain' namely the 2- and 3-methylhexanes also Yieldtoluene;

Similarly, the dimethylhexanes, namely, 23-, 2,4-, 3,4- and 2,5-dimethylhexanes, which can yield only one di-- methylbenzene(one-xylene) for a given dimethylhexane, by dehydrocyclization-in the absence of isomerization, yield the corresponding xylenes. For instance, when a vaporous feed'mixture of 82% 2,5-dimethylhexane and 18% 2,5-di-methylhexene -3 and. about 2.4 mol proportions of free iodine was passed through an unobstructed reaction zone (empty quartz tube: 4 cm. inside diameter, 811 cc. volume) atabout 500 C., at atmospheric pressure and for a nominal residence time of about 5 second, a conversion of about-63% of the 2,5-dimethylhexane wasobtained, with a yield of about 97% aromatics,

being about 96% (l -aromatics which was about 99% para-xylene. This fraction was essentially free from the isomers, orthoand meta-xylene, indicating the avoidance of any significant amount of isomerization. Furthermore, the amount of degradation was only very small, as indicated bya yield of only about 0.9% of toluene, representing only a monodemethylation, and only about 0.1% benzene, representing di-de'methylation. Similarly,

2,4-dimethylhexane yields essentially only meta xylene, and 2,3-dimethylhexane and 3,4-dimethylhexane yield essentially only ortho-xylene in the xylene fraction.

As higher homologues are reacted in accordance with 1 this inventiomthe possibilities for the production of a pluralityof individual aromatic compounds from a single 7 When normal octane, which byarom'atization without demethylation and in the absence of isomeri'zation can yield only ortho-xylene or a C -alkylbenzen'e (or C -alkenylbenze'ne), is subjected to similar conditions in the presence of a substantially chemical equivalent of free iodine, the C -product aromatic fraction is made up of orthoxylene, ethylbenzene and styrene. When the temperature ofthe process or I or total conversionis raised, the ratio of styrene to ethylbenzene increases. When ethylbenzene is subjected to similar conditions admixed with about 0.6

mole of iodine per mole of ethylbenzene, .a yield of about 90% styrene, based on the ethylbenzene converted, is.

obtained.

The dehydrocyclization-of 3-metliylheptane by reacting in the vapor phase with free iodine. at a temperature of about 500 C. yields, as C -aromatic product, a mixture of ethylbenzene, styrene and orthoand paraxylenes.- When the temperature of reaction, or 1 fed. or conversion, is

raised, the proportion of styrene in the product is substantially increased.

Of course, as is readily understood, certain higher hornologuescan give only a single aromatic by the dehydrocyclization, without degradation or isomen'zation. Thus, 2,3,4,5-tetramethylhexane yields only prehnitene; 2,3,5-

trimethylheptane yields three benzene derivatives, namely, 1

durene, 3,4-dimethylethylbenzene and 3,4-dimethylstyrene.

In addition to the dehydrocyclization 'ofcyclic hydro carbons, as shown by the preceding representativeinstances, dehydrocyclization can be effected with substances which contain cyclic structures and which" contain six contiguous carbon atoms which are not already in a singlering. Thus, arylalkanes and arylalkeneswhich contain six contiguous carbon atoms, including no more than two atoms of a single aromatic ring, such as n-butylbenzen'e,

o-diethylbenzene, o-methylpropylbenzene, and thelike, may be dehydrocyclized by the present-invention to give corresponding compounds containing an additional ring;

such as naphthalene, and the like. Similarly, alphaethylnaphthalene upon dehydrocyclization gives acenaphthene and acenaphthylene; 1,2-diethy1naphthalene gives phenanthrene; and o-methylbenzene gives'anthracene and dihydroanthracene.

As disclosed and claimed in copending application Serial No. 489,303 of applicant and Richard D.-Mullineaux, a carbon atom of a chainof at least. two carbon atoms attached directly to a quaternary carbon-atom undergoes a cleavage from the quaternary C-atom and forms a' new C-to-C linkage with another C-atom attached directly to the quaternary C-atom, thereby changing'the quaternary C-atom to a non-quaternary C-atom and lengthening the 7 Thus, 2,2,5-trimethylhex-- ane is isomerized to a 2,6-dirnethylheptane structure, with chain of contiguous C-atoms.

dehydrodemethylaromatization' to m-xylene, in addition to the conversion indicated just before.

As already described generally, the conditions for carry ing out the process of this invention are selectedsuch that, in the absence of the iodine there would be only a relatively low rate and amount of dehydrocyclization. This, of course, depends on the particular compound being converted. For the dehydrocyclization of hydrocarbons, the

temperature required is at least about 300 C., generally being at least about 350 C., and usually preferably in the order of about 425 525 C. Although, higher temperatures may be utilized'up to about 600 C., but it is usually preferred not to go above about 575 C. Higher temperatures are not objectionable so long as other undesirable changes are not effected. However, excessively high temperatures are not required in order to efiect suitabl dehydrocyclization in the presence of the relatively large proportion of iodine.

In the case of less thermally'stable substances, the temperature is more suitably adjusted within the lower range of values, such as about 400 C. to 450 C., and in some cases it may be as low as about 300 C. to 350 C. Thus, lower temperatures efiect less dealkylation, which tends to be greater'tlie more highly branched the hydrocarbon starting material, as well as the longer the hydrocarbon chain.

An important factor in the practice of theinvention is the carrying out of the conversion in the presence of a substantial proportion of free iodine. Advantageous results are obtained by the use of an amount of iodine which is of the order ofthe'chemical equivalent of the dehydrogenation to be effected. A smaller proportion may be utilized for lower conversions and yields under otherwise similar reaction conditions; In order to obtain the maximum conversion of the substance to the desired dehydrocyclized product,.two atomic proportions of iodine are required per new- C-to-C linkage. For example, in the case of n-hexane being converted to benzene, there is the cleavage of eight C-to-H linkages and the formation offour new C-to-C linkages, thus requiring eight atomic or four molecular proportions of iodine. As a practical matter aboutone-half this amount gives good results, although there is only partial conversion. The unreacted material is recoverable and can be recycled to react with a further (or regenerated) portion of iodine. Of course, an excess of free iodine can be used to insure a sufficient concentration of iodine in the vapor phase when the concentration of n-hexane has been reduced to a low value. As a practical matter, however, something less than complete conversion in a single stage represents a more realistic operation. So that even in such cases as for n-hexane, about of the theoretically chemical equivalent of iodine (mole ratio of I /hydrocarbon=3.6) is a preferred upper limit on the proportion of iodine. On the other hand, inorder for the process to have practical utility, it is required that at least about 5% of the initial hydrocarbon reactant be converted in a single pass to the desired dehydrogenation product, so that the proportion of iodine is such that twice the moles of iodine is at least 5% of the'product of the number of moles of hydrocarbon and the number of hydrogen atoms to be removed per molecule to yield the aromatic hydrocarbon. Particularly suitable mole ratios of 1 hydrocarbon are from about 0.6 to about 4, although still higher ratios can be used, such as 6-8, the'higher ratios being preferable when the hydrocarbon represents four or more equivalents in the dehydrogenation.

The process is suitably carried out at various pressures, from subatmospheric to superatmospheric pressures in vapor phase. Although atmospheric pressure is suitable and is advantageous in most cases, other considerations, such as factors which are involved in the separation and recovery of the hydrogen iodide from the product stream, in some cases make a superatmospheric pressure more desirable. Thus the'pressure. can be at any value at which thereactants are sufliciently vaporized at a temperature at which the hydrocarbon is substantially thermally stable. The pressure can be, for example, as high as from 500 to 1000 pounds per' square inch, and even higher in some cases.

The influence of the proportion of iodine on the conversion'of n-hexane can. be seen from the data'in Table I, which gives, details of'reaction conditions and of product analyses.

Table I.--Reaction of n-hexane and iodine Pressure: 1 atmosphere Temperature: 497-527 G.

Reactor: Quartz tube (4 cm. I.D.; 811 cc.) n-Hexane Fed: 1 mol Nominal Residence Time 0.5-0.8 minute Iz/n-Hexane (molar) 0. 01 0.111 M). 25 0.62 1.86 Temperature, C 527 527 527 527 497 Reacted:

n-Hexane (percent) 20 39. 6 553 49. 5 57. 7 In (percent) 100 99 98 96 10 Equivalents (percent of Reacted) Produced:

16 30 46 80 1 1 12 6 8 29 30 52 88 70 48 7 Hydrogen d 4 3 6 14 9 Hydrogen Iodide -100 93. 5 94 99.2 Total 02-0 sets/Total O olefins 0.36 1 8 6.6 7

The influence of temperature on the process will be seen from the data in Table II, for the reaction of nhexane and iodine.

Table II.-Reacti0n of n-hexane and 1;: Effect of temperature Pressure: 1 atmosphere Reactor: Quartz tube (4 cm. I.D.; 811 cc.) Nominal Residence Time: ca. 0.5 minute 1 /0 11 (molar): 0.4

Temperature, O G UI( l G) (molar) Thus, as seen from the data of Table I, as the I /C H mol ratio is raised from 0.01 to 0.6 at a constant temperature of 527 C., and a roughly constant contact time, not only does the rate of hexane disappearance rise, but the nature of the products changes profoundly. Whereas in the absence of iodine the reaction is principally degradation to smaller molecules through consecutive scission of free radicals, the addition of iodine rapidly enhances the preservation of the C structure. The use of iodine in increasing proportions not only reduces the amount of C C products but markedly increases the saturate to olefin mol ratio of these lighter products.

According to the data in Table II, reduction of the operating temperature from 527 C. to 497 C. at constant I /C H mol ratio in the feed also enhances retention of the C structure. When both reduced temperature and high I /C H mol ratio are employed, an unusually high C -structure yield is obstained and the overall reaction closely approaches the stoichiometry:

Since, as is well known, hydrogen iodide may be decomposed to produce free hydrogen and iodine, catalyzed or uncatalyzed, or may be reacted with an oxidizing agent, such as oxygen, to form free 1 the iodine can be readily regenerated and recycled to the rehydrogenation zone for further utility in the process.

It is to be seen from the data in Table I that essentially all of the iodine was converted to hydrogen iodide. Thus, this essentially complete absence of iodine does not correspond to the equilibrium concentration of free iodine for the reaction: 2HI=H +I at the reaction zone temperature. Therefore, it is advantageous to carry out the reaction, at least the later stage(s) thereof, in the presence of a catalyst for the decomposition of hydrogen iodide into hydrogen and iodine. It is thereby feasible to increase the concentration of free iodine in the system above that which prevails downstream in the reaction zone in the absence of a catalyst for hydrogen iodide decomposition, and without further addition of free iodine,

substances may be utilized, usually in less than stoichio-' metric proportions, such as oxygen (in air), nitrogen dioxide, sulfur, etc. For this purpose, the amount of oxidizing agent will usually be from about 0.1 to about 0.9

chemical equivalent of the total iodine (all forms) present.

Thus, when using free oxygen, either as such or in a mixture with an inert gas, such as nitrogen, as in air,

or carbon dioxide, etc., 8 parts by weight of oxygen is.

the chemical equivalent of 127 parts by weight of iodine, so that the oxygen used is generally from about 0.5%, to 5% by weight of the iodine (counting it as iodine in both free form and as iodide). When the iodine is proposed to be recovered fromany resulting hydrogen iodide by oxidation, there is no disadvantage to the recovery in effecting the oxidation in the reaction zone. In that case, a chemical equivalent of the oxidizing agent can be utilized. However, when it is desired to recover the hydrogen, which is removed from the hydrocarbon, as free hydrogen, as by pyrolysis of the hydrogen iodide to hydrogen and iodine, it is desirable to use the minimum amount of oxidizing agent in the reaction zone which is suflicient for the desired conversion and yield.

The products of the reaction are readily separated for the recovery of the aromatic hydrocarbon, separation of the unreacted hydrocarbon reactant for recycling, and separation of the hydrogen iodide and its reconversion to free iodine for recycling. Free iodine in the product stream can be separated with the hydrocarbons and then separated from them by a subsequent operation.

The discovery that hydrocarbonshaving chains of at least six nonquaternary C-atoms can be readily converted to aromatics by vapor phase reaction in the presence of a substantial reactive proportion of free iodine can be utilized for the large scale production of aromatics. Furthermore, the process can be applied to a mixture of such non-aromatic hydrocarbons. The resulting mixture of aromatic and unchanged non-aromatic hydrocarbons, which may have overlapping boiling points, can then be separated as by a solvent extraction or an extractive distillation utilizing a selective solvent for the aromatics.

I claim as my invention:

1. A procms for the production of aromatics by dehydrocyclization of a hydrocarbon containing at least six contiguous non-quaternary carbon atoms in a chain, no more than two of which are in a single ring, which comprises injecting said hydrocarbon into a reaction zone, heating it therein in the presence of at least about 0.6 mole of free iodine per mole of said hydrocarbon in vapor phase for a time not exceeding about 0.8 minute at a temperature of at least 300 C. to effect C-to-H bond cleavages in the molecule in the presence of free iodine, whereby said iodine is converted to hydrogen iodide and at least about 46 mole percent of the hydrocarbon converted is dehydrocyclized to an aromatic product, withdrawing the resulting reaction mixture from said reaction zone and recovering said aromatic product from said mixture.

2. A process in accordance with claim 1, wherein the hydrocarbon is an acyclic aliphatic hydrocarbon.

3. A process in accordance with claim 1, wherein the hydrocarbon is an aryl-substituted aliphatic hydrocarbon.

4. A process in accordance with claim- 1, wherein the mol ratio of iodine to hydrocarbon is from about 0.6 to about 8.

5. A process in accordance with claim 1, wherein the hydrocarbon has a chain of six carbon atoms having only hydrogen and methyl substituents on said six carbon atoms.

6. A proce'ss in accordance with claim 5, wherein'the hydrocarbon contains'only eight carbon atoms.

"7.;A process in accordance with claim 1, wherein the hydrocarbon is an x,y-dimethylhexane in which -x and y refer to difierent carbon atom positions from 2 to 5, inclusive, in the hexane chain.

8. A process in accordance with claim 1, wherein the hydrocarbon is 2,5-dimethylhexane and the recovered dehydrocyclized product is p-Xylene.

9. A'process in accordance with claim 1, wherein the hydrocarbon is n-hexane and the recovered dehydrocyclized product is benzene.

10. A process in accordance with claim 1, wherein the hydrocarbonis a heptane and the recovered dehydrocyclized product is toluene.

11'. A process according to claim 10, wherein the hydrocar'bon isn-heptane.

12. A-process in-accordance'with claim 1 wherein said hydrocarbon is a saturated hydrocarbon.

13. A process in accordance with claim 1 wherein'said hydrocarbon is an olefin.

14. A process in accordance with claim 1 wherein substantially the entire charge to the process consists of saturated'hydrocarbons containing at least six contiguous non-quaternary-carbon atoms in a chain, no more than two of which are in a single ring.

15. A' process in accordance with claim 1 wherein substantially the entire charge to the process consists of olefinic hydrocarbons containing at'leastsix contiguous non-quaternary carbon atoms in a chain, 'no'more than two of which are in a single ring.

16. A process according to claim 1, in which said resulting reaction mixture is cooled prior to recovering said dehydrocyclized product of the hydrocarbon.

References Cited in the file of this patent UNITED STATES PATENTS 1,925,421 Van Peski Sept. '5, 1933 2,259,195 Baehr et a1. Oct. 14, 1941 2,315,499 Cantzler et a1 Apr. 6, 1943 2,492,844 Condon Dec. 27, 1949 2,666,798 Condon Jan. 19, 1954 FOREIGN PATENTS 849,804 France Aug-28, 1939 OTHER REFERENCES Chemical Abstracts, vol. 25 (1931), page 5967. Bairstow et a1.: Jour. Chem.'Soc. (1933) page 1158.

Hydrocarbons from Petroleum, Rossini et a1., Reinhold Publishing Corp., N.Y. (1953), pages 407-409. 

1. A PROCESS FOR THE PRODUCTION OF AROMATICS BY DEHYDROCYCLIZATION OF A HYDROCARBON CONTAINING A LEAST SIX CONTIGUOUS NON-QUATERNARY CARBON ATOMS IN A CHAIN, NO MORE THAN TWO OF WHICH ARE IN A SINGLE RING, WHICH COMPRISES INJECTING SAID HYDROCARBON INTO A SECTION ZONE, HEATING IT THEREIN IN THE PRESENCE OF AT LEAST ABOUT 0.6 MOLE OF FREE IODINE PER MOLE OF SAID HYDROCARBON IN VAPOR PHASE FOR A TIME NOT EXCEEDING ABOUT 0.8 MINUTE AT A TEMPERATURE OF AT LEAST 300* C. TO EGGECT C-TO-H BOND CLEAVAGES IN THE MOLECULE IN THE PRESENCE OF FREE IODINE WHEREBY SAID IODINE IS CONVERTED TO HYDROGEN IODIDE AND AT LEAST ABOUT 46 MOLES PERCENT OF THE HYDROCARBON CONVERTED IS DEHYDROCYCLIZED TO AN AROMATIC PRODUCT, WITHDRAWING THE RESULTING REACTION MIXTURE FROM SAID REACTION ZONE AND RECOVERING SAID AROMATIC PRODUCT FROM SAID MIXTURE. 